The present invention relates to a process for preparing molded polymeric articles. More particularly, the present invention relates to a process for preparing molded polymeric articles exhibiting mold release characteristics from compatibilized formulations.
Polymeric moldings are being increasing used in the manufacture of automobiles, furniture, building construction materials and the like, in particular, molded polyurethanes and/or polyureas are of increasing importance because they are lightweight and resistant to moisture, temperature extremes, and deterioration due to aging. Molded elastomers have become of special interest in the manufacture of force-reducing impact media such as impact resistant automotive fascia. For some purposes, including, for example, automobile interior door panels and other automotive parts requiring particularly low density and also high strength, it has been found desirable to incorporate a blowing agent to reduce density and/or a fibrous reinforcing material, such as particulate fillers or a fibrous mat, to increase strength and dimensional stability.
The high demand for molded polyurethane and/or polyurea articles requires that these parts be produced in the largest numbers in the shortest possible time. Polyurethane-forming mixtures are generally well-suited for mass production because the reactants are liquid, and therefore pumpable, and react very quickly. A problem has existed, however, in providing adequate mold release in the shortest possible time to take full advantage of the unique capabilities of the polyurethane and/or polyurea systems.
Heretofore, release of molded articles from molds in which they have been formed has been achieved by coating the surface of the mold cavity with an agent which facilitates release of the molded article from the walls of the mold cavity. Procedures such as this are described in U.S. Pat. Nos. 3.694,530: 3,640769: 3,624,190: 3,607,397: and 3,413,390. This method has certain important disadvantages. One problem is that the externally applied release agent may, after molding, adhere to the surface of the molded article. Thus, the external mold release agent is removed with the molded part and must be reapplied after each molding, i.e., "shot". The need to repeatedly reapply the mold release agent greatly increases the time required to make each molded article, and this in turn causes a significant increase in the cost of preparing the molded part.
A second problem encountered is that the mold release agent may act to affect the surface properties of the molded part. Therefore it must, in many cases, be removed to enable further processing, such as painting or adhesion. Again, the cycle time is expensively extended.
A third problem encountered is that build-up of polymer or external mold release agent in the mold from the article being molded may occur. In this instance a film of polymer or external mold release agent may be left in spot areas of the mold's inner surface. This build-up on the surface of the mold cavity walls may eventually cover and obscure detail on the mold cavity surface desired to be imparted to the molded article.
In order to avoid the problems encountered with external mold release agents various researchers have developed "internal mold release agents". These agents are incorporated into the polymer material prior to molding, and as such avoid the need for repeated application of an external agent. Among these are, for example, primary, secondary and tertiary amine salts of carboxylic acids and those disclosed in, e.g., U.S. Pat. Nos. 3,726,952: 4,024,088: 4,098,731: 4,130,698: 4,111,861: 4,201,847: 4,220,727: and 4,585,803.
However, some of these internal mold release agents also result in processing difficulties that are at least as detrimental to total time and quality considerations than those problems caused by the use of external release agents. One problem is that some of these internal mold release agents are incompatible with polyether polyols, and thus experience limited application, and/or may seriously reduce the activity of a required reaction catalyst. Then additional means must be provided to restore the catalyst's activity. See e.g., U.S. Pat. No. 4,111,861, wherein it is noted that certain polar metal compounds, including the bismuth, lead, sodium, lithium and potassium ions, used with sodium carbonate, sodium oleate and potassium laurate are helpful. Sodium oleate alone has also been demonstrated to be an effective release agent. Finally, most of these internal mold release agents are shown to be detrimental to desirable physical properties of the final product, such as elongation.
Zinc stearate and a number of other metal carboxylates have been known to be effective release agents for thermoplastics. However, as a single additive it cannot be readily dispersed in certain polyurethane systems and therefore fails to exhibit adequate release characteristics. It was recently shown in U.S. Pat. Nos. 4,876,019 and 4,585,803 that it can be compatibilized by the presence of an amine and function as an excellent internal mold release agent in various polyurethane systems.
However, the above-noted internal mold release agents have not been used in foamed polyurethane and/or polyurea systems comprising ethylene glycol or other relatively low molecular weight polyols, in combination with relatively high molecular weight polyahls with which the polyols are normally incompatible or incompatible in the proportions employed. In these systems the incompatibility of the constituents results in a cloudy or otherwise phase-separated system that may present problems in storage stability or processing. This problem is further exacerbated when water is included in the system as a blowing agent, since the water and the relatively high molecular weight polyol are also incompatible in certain proportions. However, water is becoming an increasingly preferred blowing agent for flexible and semi-flexible foams, as many other blowing agents are discouraged due to environmental concerns. Researchers and manufacturers alike have been heretofore resigned to the need for external mold release agents in preparing these water-blown foams.
External mold release agents commonly used for these systems include, for example, solubilized hydrocarbon waxes, silicone oils, soaps in solution and the like. The external release agents generally adhere to the molded article, and thus provide only one release before reapplication is required. As already noted, this is both costly and time-consuming. In adhering to the molded article the external mold release agents may also result in surface quality and industrial hygiene problems related to the presence of environmentally-suspect solvents.
Formulations containing water as a blowing agent are also used for fiber-reinforced composites, particularly for a method of preparing fiber mat-reinforced composites known as low density reinforced reaction injection molding (RIM). Reinforced RIM molding is achieved in two ways. In one a reinforcing mat such as, e.g., a fiberglass mat, approximating the size and, in some instances, shape of the interior of the mold is prepared and then inserted into the mold. Once the mat is in place the reactant mixture, generally containing polymeric isocyanates, polyahl formulation, catalysts, surfactants and water as a blowing agent as well as, in some cases, additional blowing agents, is introduced into the mold. During the molding operation the reactant mixture soaks, surrounds and adheres to the mat material, and at the same time the water and additional blowing agents provide blowing to form a cellular composite having particularly low density. The presence of the fiber reinforcing mat provides excellent dimensional stability and increased strength with minimal increase in weight. Alternatively, reinforcing fillers such as milled glass can be incorporated into the polayahl formulation, which is then dispensed into the mold with the isocyanate, forming a dimensionally stable molded article of low density.
Again, however, those in the industry are presently using external mold release agents to prepare these foamed molded fiber-reinforced composites. As is the case with other foamed polymeric materials, external mold release agents such as solubilized hydrocarbon waxes, silicon oils, soaps in solution and the like are employed, with the usual array of associated problems as discussed above.
In view of the problems encountered with external mold release agents for molded polyurethane and/or polyurea systems which contain ethylene glycol or another relatively low molecular weight polyol and relatively high molecular weight polyahls which are incompatible in the applicable proportions, and also in some cases water, it would therefore be highly desirable to provide foamed compositions containing these starting materials and an internal mold release agent which significantly reduce or eliminate the need to apply external mold release agents and which exhibit compatibilization of the relatively low molecular weight polyol and the relatively high molecular weight polyahl and, optionally, of the water. It would be further desirable that such compositions be applicable to foamed polymeric moldings in general, as well as to fiber-reinforced polymeric moldings.